1. Field of the Invention
The present invention generally relates to semiconductor manufacturing process. More particularly, the present invention pertains to methods and apparatuses for high pressure gas annealing.
2. Description of the Related Art
During the semiconductor manufacturing process, various different thermal treatments are performed on a semiconductor wafer, for example, during or following oxidation, nitridation, silicidation, ion implants, and chemical vapor deposition processes, to achieve effective reaction with the interface as well as the bulk of the semiconductor wafer. A hydrogen or deuterium passivation process is also a known practice performed at elevated temperature, typically at around 400° C.˜500° C.
Key determining factors for effective reaction not only include the process temperature, but also the processing time and the concentration of a particular gas or a mixture of gases used for a particular application or treatment. These three factors are generally considered as independent variables which determine the efficiency of the processing. For example, by increasing the process temperature while keeping the gas concentration constant, the process efficiency will improve. Similarly, by increasing the gas concentration at the same temperature, the process efficiency can be improved. It should be noted that exposure of semiconductor wafers, or more precisely integrated circuits, to excessive heat generally degrades the quality of the integrated circuits, in an irreversible and cumulative way. This is, in part, due to the diffusion of various carriers and ions implanted on the wafer, whose rate increases, typically superlinearly, with temperature. Each integrated circuit has an acceptable limit of total thermal exposure during the whole manufacturing process, which is referred to as the circuit's thermal budget in the related art.
As the technology and device structure approaches the nanometer scale, the limited thermal budget requirement demands higher concentration of the processing gas and/or lower treatment temperature. However, increasing the gas concentration and lowering the processing temperature has its own limitations due to the efficiency problem at lower temperature as well as to the safety problem caused by high concentration gases. Even though, by increasing the processing time without changing the temperature and gas concentration parameters, comparable process efficiency can be achieved, the processing time and the temperature are considered together as a part of the thermal budget limitation. That is, increasing the processing time has a similar (adverse) effect on the device's performance, for example, as elevating the process temperature.
Annealing wafers in a forming gas containing diatomic hydrogen, typically following fabrication but before encapsulation or other packaging steps, has been widely used for repairing various process induced damages during the semiconductor fabrication process as well as for sintering process, which is referred to as hydrogen passivation in the art. The annealing or forming gas generally incorporates approximately 2% to 10% hydrogen (H2) with the remainder being inert gas such as nitrogen (N2). Recently, however, many research reported that pure (100%) hydrogen or deuterium anneal improves the device characteristics and performance such as hot carrier reliability, transistor lifetime, and reduction of dangling bonds and unwanted charge carriers. Improvement of device lifetime increases the transconductance (speed performance) of the device. As the device technology and structure move to the sophistication of the so-called “nanometer technology”, new high pressure application technologies require use of other gases such as fluorine (F2), ammonia (NH3), and chlorine (Cl2), which can be highly reactive or toxic. The forming gas (partial pressure) anneal and/or pure H2 or D2 anneal has been generally done at a temperature range above 450° C., and higher temperature tends to result in better performance. However, as the device scale reaches below 0.12 μm range, the limited thermal budget after first metallization requires annealing temperatures at or below 400° C., thus potentially diminishing the hydrogen annealing benefit on semiconductor device performance.
As an alternative, hydrogen or deuterium high pressure anneal has been proposed, and some excellent performance improvement has been reported. Particularly, hydrogen and/or deuterium anneal of high-K gate dielectric device showed significant performance improvement in charge reduction, dangling bond reduction, and increase of transconductance. This finding has been disclosed, for example, in the U.S. Pat. Nos. 6,913,961 and 6,833,306. This improvement is very significant for the manufacturing process of integrated circuit devices using high-K gate dielectric for the next several generations of semiconductor device technology.
One of the main advantages of the high pressure technology is the increase of the reaction rate by effectively increasing the gas concentration at high pressure. By increasing the pressure of the processing gas, the density of the processing gas will increase. The gas density increases roughly linearly as the pressure increases. For example, if pure 100% hydrogen is processed in 5 atm high pressure condition, the actual amount of hydrogen gas that semiconductor silicon is exposed to is 5 times the concentration of the original (100%) hydrogen gas at the atmospheric pressure. In the case of partial pressure conditions, if the hydrogen concentration is 20% and the silicon wafer is processed at 5 atm pressure, then the silicon wafer is effectively exposed to the equivalent of 100% hydrogen at atmospheric pressure. Likewise, processing with 20% hydrogen gas at 20 atm will be roughly equivalent to 4 times of the processing result with the pure (100%) hydrogen gas at 1 atm.
By increasing the pressure of the process gas, it is possible to reduce both the processing temperature and the process time. As the thermal budget limitation reaches the “extreme limit level,” and as the device technology reaches the 45 nm range, high pressure processing becomes the only viable solution which meets or exceeds many thermal processing requirements in the semiconductor fabrication technology. The high pressure processing can provide the following benefits with respect to the three aforementioned process parameters; process time reduction, process temperature reduction, and process gas concentration reduction. (1) By increasing pressure, the process temperature can be reduced while maintaining the gas concentration and process time unchanged in order to obtain equivalent or similar process results. (2) By increasing pressure, the process time can be reduced significantly while keeping other parameters of temperature and gas concentration unchanged in order to obtain equivalent or similar process results. (3) By increasing pressure, the process gas concentration can be reduced while maintaining the time and temperature parameters unchanged in order to obtain equivalent or similar process results.
Application of high pressure hydrogen/deuterium process anneal to high-K gate dielectric process anneal, post-metallization sintering anneal, and forming gas anneal in the semiconductor fabrication could achieve a significant improvement in the device performance, for example in terms of increased device lifetime, enhanced transconductance, and reduced number of dangling bonds, and also achieve significant process thermal budget improvement at a given processing temperature and processing time, which is an essential requirement for the advanced device technology.
Although significant performance enhancement of semiconductor device by high pressure anneal, in particular, in the hydrogen (H2) or deuterium (D2) environment, has been known for some time, the semiconductor manufacturing industry has been unable to implement this fabrication technology in the production line. One of the main hindrances has been various safety concerns with existing annealing systems and, in particular, high pressure processing vessels.
Some of the processing gases are highly reactive, inflammable, toxic, or otherwise dangerous, and when these gases are pressurized, the likelihood of leakage of the gas from the pressure vessel or its support subsystems to the atmosphere increases, and the danger becomes far greater. Hydrogen/deuterium gas, for example, is highly inflammable, and when high concentration of hydrogen/deuterium is exposed to oxygen in the atmosphere, it can explode. Furthermore, due to its small molecular size, there is higher likelihood of leakage of hydrogen/deuterium under high pressure conditions. This has resulted in slow development of high pressure H2/D2 processing vessels in the semiconductor capital equipment industry. The same has been true with usage of other toxic, inflammable, or otherwise dangerous gases in high pressure annealing environment.
A high pressure processing system typically includes the following three main segments or subsystems: (1) A high pressure gas process vessel, (2) an incoming high pressure gas delivery system, and (3) a disposal (venting) mechanism of the high pressure gas after the process completion. Of these, the most important will normally be the process vessel or chamber, where the process gas such as hydrogen is being pressurized and semiconductor wafers are being processed. The door, or cover, of the chamber is typically sealed with O-rings. Under high pressure and high temperature condition, if the O-ring is unable to hold its seal, the process gas such as hydrogen can leak to the atmospheric air. Any other joints in the assembly or defects in the material could also result in leakage of the gas, leading to potentially dangerous situation.
Another safety issue with a high pressure hydrogen tool is the incoming gas control subsystem or module, where flow-meters, mass-flow controllers, and other gas control mechanisms are connected. These connection joints are all potential sources of hydrogen leak, and any weakness or failure at these joints could create a hazardous condition. The stainless steel pipe used to transport the process gas is also a part of the safety concerns. Even though the quality of stainless steel pipe today is generally very high, any defect in the stainless steel itself can result in leak of hydrogen under high pressure conditions.
Another important element to consider in designing high pressure annealing systems is how to exhaust the process gas such as highly pressurized hydrogen or deuterium into the atmosphere when the processing is done. The pressurized hydrogen could be up to 100% in concentration, and could create a safety problem unless it is properly handled. Another safety issue is the load-lock environment where the door of the process chamber opens after the process is completed and gases are depressurized. Even after the process gas is depressurized, there is always a possibility of residual gas being trapped in the chamber, and this residual gas such as hydrogen could interact with the atmospheric air.
Partly due to these safety concerns, even though it has been well known that high pressure hydrogen process is highly beneficial for semiconductor device performance, the capital equipment industry has been reluctant to invest the capital necessary for developing high pressure hydrogen process equipment. Most of the reported high pressure hydrogen processing tools are prototypes used, or tested, only in the laboratory settings, one of which is shown in FIG. 1A. The figure illustrates a cross-sectional view of a typical high pressure hydrogen processing vessel in the prior art. It should be noted that the vessel shown in the figure is a prototype, or a research lab model. This particular design of the high pressure process vessel may not be used in the production environment of the semiconductor fabrication process due to the aforementioned safety issues.
FIG. 1A shows a main processing vessel 10, made of stainless steel in this case, and a vessel cover 11 on top of the vessel. The cover, when closed, is typically sealed with an O-ring (not shown in the figure). Inside the main vessel 10, a heater 20, a reactor vessel 21, and the reactor cover 22 are stacked together to hold a semiconductor wafer (not shown). Through the top vessel cover 11 to the inside of the main vessel 10 are connected H2/D2 gas injector or inlet 15, a pressure sensor 14, and gas exhausting outlet 16. The main heater is connected with temperature sensor/thermocouple 17 and thermocouple 18, both of which are connected to a temperature controller 12. The pressure sensor 14 is then connected to a pressure monitor unit 13, which is in turn coupled to the temperature controller.
The design of the prototype shown in FIG. 1A does not take into consideration the safety issues mentioned earlier, and this particular model is designed for feasibility study only. It should be noted that this vessel is designed to hold a single wafer only. The system is operated by first mounting a wafer into a reactor vessel 21. The wafer is heated by the heater 20 and the hydrogen gas is introduced from the top through the gas injector 15. The pressure sensor 14 and the pressure monitor unit 13 monitors the gas pressure inside the main chamber, while the thermocouples 17 and 18 monitor the temperature. Once the desired gas pressure and temperature is reached, a typical annealing process lasts about half an hour. After the process is completed, the H2 gas in the chamber is exhausted via the outlet 16.
One of the weakest parts of this design would be the main O-ring used to seal the process chamber or vessel. This is depicted in FIG. 1B, where the main O-ring is explicitly shown. The figure shows a cross-sectional view of a main vessel 110 and its cover 111, as in FIG. 1A. The vessel has a cylindrical shape in this example. Other parts included in FIG. 1A such as incoming gas lines and exhaust pipes are not shown in FIG. 1B for the sake of clarity. In this illustration, the gap between the vessel 110 and the cover 111 is sealed with an O-ring 109. An O-ring has a shape of a circular ring and it typically has a circular or elliptical cross section. The figure shows an O-ring with a rectangular cross section. If this main O-ring gives away to a high pressure, the hydrogen gas can leak out of the vessel and can ignite when exposed to oxygen in the atmosphere. This can create an extremely hazardous condition. Even under normal operating conditions, there is a higher likelihood of leakage of hydrogen around the O-ring.